Mechanical Engineering DepartmentFaculty of Engineering Kabul University 2 | P a g e Abstract of Project Scissor jacks are simple mechanisms used to drive large loads for short distances. The power screw design of common scissor jack reduces the amount of force required by the user to drive the mechanism. Most scissor jacks are similar in design, consisting of four main members driven by a foot paddle. In this report, a unique design of a scissor jack is proposed which is very easy to manufacture. Each member, including the power screw sleeves, is made of common c-shape. This eliminates the need for machined power screw sleeves, which connect the four members and the power screw together. The manufacturability of the proposed scissor jack lowers the cost of production. Scissor jacks allow raising heavy loads using only a fraction of the force ordinary needed. In this project the effort was to design an efficient scissor jack to operate by leg power capable of raising a 2000 KG load 3 | P a g e Contents Introduction Scissor jack Components (tools) Preliminary Designs Load criteria and assumptions: Material selection: Design phase Models ANALYSIS Possible failures and errors References Thus we conclude 4 | P a g e Introduction A jack is mechanical device used to lift heavy loads or apply great forces. Jacks employ a screw thread or hydraulic cylinder to apply very high linear forces. A mechanical jack is a device which lifts heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Car jacks usually use mechanical advantage to allow a human to lift a vehicle by manual force alone. More powerful jacks use hydraulic power to provide more lift over greater distances. Mechanical jacks are usually rated for a maximum lifting capacity. As our area of concern is a screw jack used for lifting the car that is scissor jack, so only the scissor jack and its background is discussed below. Scissor jack Scissor jacks are simple mechanisms used to drive large loads short distances. The power screw design of a common scissor jack reduces the amount of force required by the user to drive the mechanism. Most scissor jacks are similar in design, consisting of four main members driven by a power screw. A scissor jack is operated simply by turning a small crank that is inserted into one end of the scissor jack. This crank is usually "Z" shaped. The end fits into a ring hole mounted on the end of the screw, which is the object of force on the scissor jack. When this crank is turned, the screw turns, and this raises the jack. The screw acts like a gear mechanism. It has teeth (the screw thread), which turn and move the two arms, producing work. Just by turning this screw thread, the scissor jack can lift a vehicle that is several thousand pounds. Power screw in a scissor jack is the foundation of whole mechanism of scissor jack. The most common problem encountered while using scissor jack is the instability of jack while giving jerks to loosen the wheel nut. Also the common jack having 5 | P a g e small base is unable to provide proper support on uneven surface esp. off-road and no inclination in that jack is tolerable. The project relates to the designing of simple scissor jack and its analysis along with structural improvements to make such a modified jack that is very stable and can take enough load on uneven surfaces and somewhat inclination is also allowed. The project also aims at designing and finding stresses, efficiency, expected life of screw. We endeavor to develop a screw jack such that it is cost effective, having a long life and can be handled roughly. Here we outline the timeline for the completion of various aspects for the project. The schedule is set so that the project is completed in phases. Phase I is market research, Phase II consists of the design process, Phase III entails PRO-e modeling of the design and simulation in ANSYS software, and the final aspect of the project is the presentation and the work that went into it. 6 | P a g e Scissor jack: Specifications The term "scissor jack" describes a wide variety of tools that all follow the same principle: using crossed beams to lift something. They do this by acting on the object they are lifting in a diagonal manner; the lift on the right side lifts the object from its left side and vice versa. This allows the user to store the jack when it is not in use (with the diagonal beams flat) and to expand it when it is needed. The major specification of scissor lifts is that they are all symmetrical. In order to work, the distance from the loaded point to the cross point must be the same as the distance from the cross point to the ground. This ensures that weight is distributed equally throughout the scissor lift beams. Since scissor lifts have such a wide variety of use, they also have a wide variety of power sources. Scissor lifts for lifting cars can be powered electrically, hydraulically and of course mechanically. On the other end of the spectrum, industrial scissor lifts that people stand on are often powered by diesel, although electrical options do exist. Scissor lifts basically fall into two categories: single scissor lifts and multiple scissor lifts. A single scissor lift has just two crossbeams and one "x." This means it 7 | P a g e can only go so high because the length of the crossbeams restricts the height of the lift, and making them too long would make it unstable. On the other hand, multiple lifts have beams crossing each other, and then attaching to more beams that go the opposite direction. This allows the scissor lift to rise higher. A scissor jack has four main pieces of metal and two base ends. The four metal pieces are all connected at the corners with a bolt that allows the corners to swivel. A screw thread runs across this assembly and through the corners. As the screw thread is turned, the jack arms travel across it and collapse or come together, forming a straight line when closed. Then, moving back the other way, they raise and come together. When opened, the four metal arms contract together, coming together at the middle, raising the jack. When closed, the arms spread back apart and the jack closes or flattens out again. A scissor jack uses a simple theory of gears to get its power. As the screw section is turned, two ends of the jack move closer together. Because the gears of the screw are pushing up the arms, the amount of force being applied is multiplied. It takes a very small amount of force to turn the crank handle, yet that action causes the brace arms to slide across and together. As this happens the arms extend upward. The car's gravitational weight is not enough to prevent the jack from opening or to stop the screw from turning, since it is not applying force directly to it. If you were to put pressure directly on the crank, or lean your weight against the crank, the person would not be able to turn it, even though your weight is a small percentage of cars. 8 | P a g e Components (tools): 1 - Frame 2 - Power screw 3 - Rivets 4 - Coupling nut 5 - Crank 1 Frame: The entire frame of the scissor jack consists of links (top and bottom), base frame, support frame. The frame is manufactured by sheet metal processes and forming by low-medium carbon steel. 2 Power screw: Power screws are used to convert rotary motion in to translational motion. It is also called translational screw. They find use in machines such as universal tensile testing machines . The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases. For example, anticlockwise forces are applied to the screw (which would work to 9 | P a g e undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. Power screws are typically made from carbon steel, alloy steel, or stainless steel and they are usually used with bronze, plastic, or steel mating nuts. Bronze and plastic nuts are popular for higher duty applications and they provide low coefficients of friction for minimizing drive torques. There are important terms and figures that need to be understood before designing power screws: 1. Pitch: is the distance from a point on one thread to the corresponding thread on the next adjacent thread, measured parallel to the axial plane. 2. Lead: is the distance the screw would advance relative to the nut in one rotation. For single thread screw, lead is equal to pitch. 3. Helix Angle: is related to the lead and the mean radius by the equation below; 10 | P a g e Basics of power screws Power screws provide a compact means for transmitting motion and power. They are ideal for replacing hydraulic and pneumatic drive systems as they require no compressors, pumps, piping, filters, tanks, valves or any other support items required by these systems. Also, screws don't leak so there are no problems with seals which are so common to hydraulic and pneumatic systems. And, screw systems are quiet running - no noisy compressors, pumps or exhaust valves. Screw systems are simple, reliable and easy to utilize. Power screw motions There are four distinct motion converting actions that can be produced by power screws and nuts. The two most common involve torque conversion to thrust. In Figure 1, the screw is rotated (torqued) and the nut moves linearly producing thrust or the nut is rotated (torqued) and the screw moves linearly. The two less common motions involve thrust conversion to torque. In Figure 2, the nut undergoes a linear force (thrust) and the screw rotates or the screw undergoes a linear force (thrust) and the nut rotates. These two motions are commonly referred to as "back driving", "overhauling", or, improperly, "reversing". 11 | P a g e Fig1. Fig2. 3 Rivets: A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre- drilled hole, and the tail is upset, or bucked (i.e. deformed), so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place. To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head or buck-tail. 4 Coupling nut: A coupling nut is a threaded fastener for joining two male threads, most commonly threaded rod. The outside of the fastener is usually a hex so a wrench can hold it. Variations include reducing coupling nuts, for joining two different size threads; sight hole coupling nuts, which have a sight hole for observing the amount of engagement; and coupling nuts with left-handed threads. 5 Crank: is an arm keyed at right angles to the end of a shaft, by which motion is imparted to the power screw .It mainly suffers from torsional stresses so medium 12 | P a g e carbon steel is used as it combines merits of malleability and sufficient torsional strength. Preliminary Designs: As stated before, the basic design and mechanics of the scissor jack are simplistic and lend little room for drastic change, so any change will be a modification on this base model. Below is preliminary design concepts sketched Load criteria and assumptions: The load for which the jack is to be employed has to be considered first. For very heavy loads we have to deal with heavy duty jacks and in those situations scissor jacks do not work efficiently and most probably fail. While in case of low and 13 | P a g e medium intensity loads, scissor jack works efficiently and smoothly without much effort. Also the jack is handy enough to carry in the vehicle. So considering the above situation, making a scissor jack for low and moderate dead loads will be a good idea. Estimated vehicle weight: 2000 kg Weight on one side: 2000/4: 500kgs. Factor of safety: 4 Weight for which is designed: 2000kgs. Material selection: Secondly, the problem of material selection is solved by selecting some materials on the basis of their strength and modulus of elasticity. We here compared mild steel , aluminum , plain carbon steels and alloy steel, stainless steel and got an overall result for the best fit material to be low-medium carbon steel .(comparison on basis of data given in MATERIALS AND HEAT TREATMENT PROCESSES by O.P. KHANNA) The material will be designed completely using plain carbon steel. Designing a scissor jack using plain carbon steel is a work of sheet metal shop. To overestimate the safety we will use calculations of strength using the plain carbon steel in its undisturbed, solid form. Low-medium carbon steel will be used 0.29% to 0.54% carbon –e.g. AISI 1040 steel Medium carbon steels can be heat treated to have a good balance of ductility and strength. These steels are typically used in large parts, forgings and machined components. MATERIAL PROPERTIES at 25c : low-medium carbon steel Density = 7845kg/m3 14 | P a g e Young’s modulus (E)=200 GPa Ultimate shear strength= 57420 PSI=342.4 MPa approx. 66% of the UTS(87000 PSI=518.8 Mpa) Yield strength= 52500 PSI =353.4 MPa Design phase Stresses acting on various components 1. Torsional stress acting on power screw. ≥ = πd 4 32 = max torsional stress = Torque d = screw diameter 2. Buckling load acting on lifting frame. ≤ 2 ( ) 2 W=axial load on frames L=length of frame C=1for long columns 15 | P a g e K= radius of gyration 3. Yielding stress acting on lifting frame. ≤ = yielding stress = endurance limit = Factor of safety 4. Bearing stress acting on rivets. ≤ σ p = yielding stress = endurance limit = Factor of safety 5. Shear stress acting on rivets 16 | P a g e ≤ .577 / = endurance limit = Factor of safety = ℎ 6. Bending stress acting on coupling joints. = m∗ 2 = ( 4 − 4 ) 4 = polar moment of inertia = outer radius = inner radius m = bending moment d = average diameter = (r+R)/2 = bending stress Self -locking criteria ≥ f = coefficient of friction d = diameter of screw 17 | P a g e 1 Models 1.1 Parts Base frame Bottom link 18 | P a g e Bottom packing Bottom rivet 19 | P a g e Link rivet Screw shaft 20 | P a g e Coupling nut Top link 21 | P a g e 1.2 Assembly Support frame Closed assembly 22 | P a g e Open Assembly 23 | P a g e ANALYSIS Following are the images of stress concentration during analysis of scissor jack. The bar on the side of the images shows the value of stress relative to its color as color progresses from blue to red, stress conc. Increases thereby making it prone to failure. So as we see there is a scope of improvement. Now we will try to design a modified jack making it safer. Bottom frame 24 | P a g e Bottom link 25 | P a g e Support frame 26 | P a g e Top rivet 27 | P a g e Top rivet 2 28 | P a g e Top packing 29 | P a g e Top link 1 30 | P a g e Top link 2 31 | P a g e Power screw 32 | P a g e Bottom rivet 1 33 | P a g e Bottom rivet 2 34 | P a g e Coupling nut 35 | P a g e Bottom link 2 36 | P a g e Possible failures and errors: A. Unstable center of gravity (Remedy: Weighted rear support brace for balance and lengthened front ¼ floor plates extending under car.) B. Jack failure due to excess mass being lifted(>2440kgs) C. Failure of primary bolts due to bending moments and shear stresses. References: Books referred Finite element Strength of materials Websites referred google.com Wikipedia.com Howstuffworks.com Sciencedirect.com B2bhydrualicjacks.com mysite.du.edu 37 | P a g e Thus we conclude “Impossible is nothing its just the mind perception and based on root analysis of data” minor project proved to be most valuable in terms of teamwork and management to us. Also we explored new territories in technical creation. We faced new challenges while designing and analyzing scissor jack by pro-engineer and inventor. The experience gained has provided us confidence in dealing with practical aspects of engineering and will prove to be invaluable as we go into placement season.